Method and system for laser amplification using a dual crystal Pockels cell

ABSTRACT

A system for laser amplification includes a dual-crystal Pockels cell which is used to control emission of laser pulses from an ultra-fast laser. The Pockels cell is constructed to enable adjustment of the rotational orientation of one crystal relative to the other crystal. The rotational orientation of one or both crystals in the Pockels cell is adjusted to control sidebands in the laser pulse.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to laser amplification and opticalswitching systems, including but not limited to Pockels cells used forcontrolling light in lasers, optical switches, and other applications.

2. Description of Related Art

Electro-optical materials operate to change the polarization of a lightbeam in response to the application of an electrical voltage across thematerial. These materials are often used in combination with polarizersas electro-optical switches. In lasers or other optical systems,electro-optical materials are often configured as Pockels cells.Depending on the type and geometry of electro-optical material and thelevel of applied voltage, the polarization of a light beam can be variedto selectively pass a polarizer which has a predetermined polarizationorientation. Thus, the transmission of a beam can be controlled asdesired by application of a voltage.

Various different configurations of Pockels cell are known in the artfor amplification of laser light, optical switching, and otherapplications. In one application for laser amplification, a laser mediumin a regenerative amplifier cavity is pumped to generate an excess ofexcited atoms in the medium. A Pockels cell is then activated to capturea seed pulse in the cavity. The seed pulse is amplified by repeatedlypassing through the laser medium. After a period of time, voltage isremoved from the Pockels cell, thereby changing its polarization andcausing the amplified pulse to be emitted from the cavity.

Dual-crystal Pockels cells with thermal compensation based on transverseeffect are known in the art for providing optical switching usingreduced control voltages. The dual-crystal Pockels cell uses twocrystals in series, which reduces the magnitude of the applied voltagerequired to activate the cell. These are usually biaxial crystals andcompensation is made for natural birefringence, which usually has astrong thermal dependence, by specially orienting the two crystals suchthat the beam passes along the X axis (for X-cut crystals) or the Y axis(for Y-cut crystals). Input beam polarization is directed at 45° withrespect to the Y and Z axes, or alternatively the X and Z axes,depending on the crystal cut. The second crystal is rotated so that theZ axes of the two crystals sit at 90° relative to each other. The twocrystals are also generally polished together to have matched lengths.The remainder of the discussion below assumes use of biaxial Y-cutcrystals, with the understanding that the entire discussion equallyapplies to biaxial X-cut crystals.

The two crystals share a common central electrical contact. Voltage isapplied between the common center electrode and the end electrodes,resulting in additive polarization change. Pockels cells of dual-crystaldesign have proven useful for Q-switched lasers where pulses in therange of 1 ns to 1000 ns are commonly generated, and for regenerativeamplifiers in lasers where pulses in the range of 5 picoseconds to 1000picoseconds are commonly generated. Short pulse widths in the range ofabout 30 femtoseconds to about 5 picoseconds are desirable for manyapplications, such as surgery or micro-machining, to precisely ablatetargeted areas without damaging surrounding material.

Dual crystal Pockels cells are customarily constructed so as to ensurethat their crystal structures are aligned. The pitch and yaw of the twocrystals, i.e., their rotational orientation for Y-cut crystals withrespect to the Z and X axes, respectively, is controlled using mountingfixtures to ensure that the Y axes of the two crystals are parallel. Theextinction ratio of the dual-crystal Pockels cell depends on theprecision with which the Y axes of the two crystals are parallel. Thedual crystals are customarily factory-installed in a structure so as toachieve the desired axial alignment, and locked into position. Normally,this alignment is fixed and not adjustable. The amount of precision inthis alignment, however, is directly related to the extinction ratio ofthe emitted pulse.

In addition, the crystals in a dual-crystal Pockels cell are rotatedaround their Y axes with respect to one another to achieve thermalcompensation. The amount of rotation is nominally 90°, such that onecrystal of the pair is rotated around its Y axis by this amount,relative to the Z (or X) axis of the other crystal. Again, the crystalsare normally factory-installed with this rotational offset, and it toois not normally adjusted during operation of the Pockels cell. Theprecision of this rotational offset is generally about ±60 minutes.

Notwithstanding the advantages of dual crystal Pockels cells and systemsthat employ them, these systems are subject to certain disadvantages,notably when used in regenerative amplifiers for amplification of laserpulses of about 30 femtosecond to about 5 picoseconds. Laser pulses fromhigh repetition rate lasers tend to occur with sidebands, which reducesthe peak laser intensity. These sidebands may not be observable forslower pulses, for example, pulses in the 5 picosecond or longer range.However, such sidebands become readily apparent for laser pulses on theorder of picoseconds or shorter. Control of these sidebands, whether toincrease the intensity of the peak pulse or for more control over theshape of the peak pulse and the resulting sidebands, is desirable.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for altering pulsesidebands in a laser amplification system that uses one or moredual-crystal Pockels cells to control the amplification in a laser. Theinvention further provides a method and system for rotationalorientation of electro-optical elements in an optical switch, such as,for example, the crystals in a dual-crystal Pockels cell. The inventionmay also be applied in other optical systems using a dual-crystalPockels cell or comparable optical switch to switch ultra-fast laserpulses.

According to an aspect of the invention, a dual-crystal Pockels cell isprovided with an adjustable mount which permits at least one of thecrystals in the pair to be rotated a small amount relative the opticalaxis (also referred to as the Y axis). The mount may also orient andalign the crystals with respect to the X and Z axes. Advantageously, themount is configured to be adjustable during operation of the opticalsystem it is to be used with. For example, for a Pockels cell that isused as part of a regenerative laser amplifier, the mount is preferablyconfigured to be adjustable while the laser is operating to enable finetuning of the amplifier on a system-by-system basis.

In an embodiment of the invention, the crystals are mounted againstfixed reference surfaces of a mount for X axis and Z axis alignment. Themount sets the relative angular displacement of the crystals about the Yaxis, i.e., the Z axis of one crystal has an angular offset with respectto the Z axis of the other crystal, to a predetermined nominal amount.For example, the crystals may be set to have a relative angulardisplacement of 90°. The mount is further configured to allow fineadjustments in the amount of relative angular displacement between thetwo crystals. Various mount configurations may be suitable. The Pockelscell with its adjustable mount is then deployed in the desired opticalsystem.

The adjustable mount can be used to reduce, eliminate, or otherwisealter sidebands that are observed in ultra-fast laser pulses switchedthrough prior-art Pockels cells. To do so, the pulse shape is observedas the rotational alignment of the crystals about the Y axis is adjustedusing the adjustable mounting system. The sidebands of the laser pulse,through proper rotational orientation of the Pockels cells, may beminimized, eliminated, or set to a desired level. The relativerotational alignment of the two crystals is then maintained duringoperation of the laser or other optical system.

According to the foregoing, therefore, an optical switch comprises afirst mount holding a first electro-optical element and a second mountholding a second electro-optical element in optical alignment with thefirst electro-optical element. The first and second mounts are adaptedto angularly position the first and second electro-optical elementsabout an optical axis and to angularly displace the electro-opticalelements with respect to one another by an offset angle about theoptical axis. An angular adjustment device is operably associated withthe second mount and is adapted to adjust the offset angle by anadjustment amount.

In addition, the method comprises placing first and secondelectro-optical elements into an optical cavity, each electro-opticalelement being angularly displaced with respect to the other by an offsetangle about an optical axis of the optical cavity. A seed pulse isdirected into the optical cavity. The electro-optical elements arecontrolled such that the seed pulse is first amplified then emitted fromthe optical cavity as a laser pulse. The intensity profile of the laserpulse is observed, and the offset angle is adjusted so as to adjustsidebands observed in the intensity profile.

A more complete understanding of the system and method for reducing oreliminating sidebands in an optical system will be afforded to thoseskilled in the art, as well as a realization of additional advantagesand objects thereof, by a consideration of the following detaileddescription of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is made to the appended sheets of drawings In the drawings,wherein like reference numerals refer to similar components:

FIG. 1 is a block diagram showing a laser system incorporating adual-crystal Pockels cell according to the invention;

FIG. 2 is a diagram illustrating alignment of a crystal pair in aPockels cell;

FIG. 3 is a block diagram showing steps for configuring a dual-crystalPockels cell;

FIG. 4 is a diagram illustrating an improvement in pulse profileachieved using a dual-crystal Pockels cell according to the invention;

FIG. 5 is a perspective view showing a first embodiment of adual-crystal Pockels cell mount with Y axis rotational adjustment; and

FIG. 6 is a perspective view showing a second embodiment of adual-crystal Pockels cell mount with Y axis rotational adjustment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a system and method for reducing,eliminating, or otherwise altering sidebands in a laser pulse system.The invention is useful in optical systems using multi-element opticalswitches, for example, dual crystal Pockels cells, for switching ofultra-fast laser pulses.

FIG. 1 shows a laser system 10 employing an optical switch comprising anadjustable Pockels cell 18 having two electro-optical crystals 19 a, 19b. The gain medium 12 is disposed within the optical cavity of thesystem 10, the optical cavity being generally defined by two mirroredsurfaces 15 a, 15 b. The gain medium 12 is adapted to amplify a seedpulse 11 using pump power received from a pump laser 13 or otherappropriate power source. The seed pulse 11 is injected into the opticalcavity from a seed laser 14. The Pockels cell 18, in conjunction withthe polarizer 16, controls emission of an output pulse 17 from theoptical cavity. The system may comprise various other electronic oroptical elements as known in the art. The invention is not limited toany particular configuration of laser system, and may be employed withany laser system making use of a multiple-element optical switch forcontrol of pulse amplification.

FIG. 2 shows an arrangement of electro-optical crystals in adual-crystal Pockels cell 20. A first crystal of the cell is indicatedschematically at 22. A second crystal of the cell is indicatedschematically at 24. X and Z reference axes are indicated for eachcrystal. Light passing through each crystal is polarized at a 45° anglerelative to the X and Z axes. The crystals 22, 24 are rotated to have arelative angular displacement with respect to each other. This relativeangular displacement is characterized by the offset angle, ω, and isusually approximately 90°. With such an orientation, the crystals 22, 24enable additive polarization. The crystals 22, 24 are aligned such that,if placed in an optical cavity, the optical axis of the optical cavitypasses through both crystals and is parallel to the Y axis of eachcrystal. By way of example, the first crystal 22 may be aligned with thesecond crystal 24 by a rotation ‘r_(x)’ around its X axis and a rotation‘r_(z)’ around its Z axis. These rotations, sometimes referred to as“pitch” and “yaw” of a crystal, serve to ensure that the Y axes of thecrystals 22, 24 are parallel. The precision with which this alignment isachieved is indicated by the Pockels cell's extinction ratio. Thedesirability of a precise alignment around the crystals' X and Z axesand of a correspondingly high extinction ratio is recognized in the art.In addition, Pockels cell 20 is configured for fine angular adjustmentrelative to the Y axis (‘r_(y)’ rotation) as further described below.

A process 30 for configuring a regenerative laser using a multi-elementoptical switch, such as a dual-crystal Pockels cell, are shown in FIG.3. The process 30 may be used with the laser systems and Pockels cellsdisclosed herein. At step 32, the laser system is set up and operatednormally to produce a laser pulse. The set-up includes placing first andsecond electro-optical crystals into the optical cavity, eachelectro-optical crystal being angularly displaced with respect to theother by an offset angle about the optical axis of the optical cavity.The seed pulse is directed into the optical cavity, and theelectro-optical crystals controlled such that the seed pulse is firstamplified then emitted from the optical cavity as a laser pulse.

The laser pulse is directed toward a suitable optical sensor responsiveto the emitted pulse. At step 34, the sensor data is processed toprovide an intensity profile for the laser pulse, using any suitableprocessing system as known in the art. An intensity profile is provided,for example, as an output waveform on any suitable media. FIG. 4 showstwo waveforms 40, 42 such as may be observed. Time in picoseconds isindicated on the horizontal axis, and intensity is indicated on thevertical axis. The first waveform 40 shows a laser pulse with sidebands41, such as may be observed from a laser system using a dual-crystalPockels cell prior to rotational adjustment. The second waveform 42shows an adjusted intensity profile, such as may be achieved afterrotational adjustment of the Pockels cell crystals. For illustrativeclarity, the vertical scale for the second waveform 42 is shiftedupwards. In addition to minimizing or eliminating the sidebands,rotational adjustment of the Pockels cell crystals about the Y axis mayalso be used to tailor the intensity profile to a desired waveform. Thismay include increasing the intensity of the sidebands to effectivelycreate a series of multiple pulses.

Referring again to FIG. 3, while observing the pulse intensity profile,at least one crystal of the dual-crystal Pockels cell is adjusted bybeing rotated about its Y axis, which is aligned with the optical axisof the optical cell. The amount of adjustment may be much smaller thanthe Z axis offset of the crystal pair. For example, a crystal of thepair might be adjusted within 10 minutes of arc, compared to a Z axisangular offset of 90°. Preferably, the Y axis rotational adjustment isaccomplished without changing the orientation of either crystal relativeto its X or Z axes, that is, their pitch or yaw. The direction ofadjustment may be determined by observing the waveform of the laserpulse. To minimize or eliminate the sidebands, the adjustment is made inthe direction that causes a reduction in the amplitude of the sidebands.As indicated at step 38, steps 32, 34 and 36 may be repeated until aclean waveform having significantly reduced or substantially nosidebands is observed for the laser pulses.

FIG. 5 shows an exemplary mounting system 100 for a dual-crystal Pockelscell adapted to minimize sidebands in a laser pulse by permitting fine Yaxis rotational adjustments while maintaining X axis and Z axisalignment. Structural components of the system generally comprise athermally stable, conductive metal such as a nickel alloy or othersuitable material. The system also comprises dual electro-opticalcrystals 106, 116 as known in the art. Suitable materials for thecrystals 106, 116 may include, for example, LiNbO₃, KTP, RTP, RTA. Otherdetails of the system 100 may be as known in the art.

The system 100 comprises a base 110 supporting a fixed mounting block102 and an adjustable mounting block 112. The system 100 may becontained inside of a housing (not shown). An electrode 124 for applyinga voltage to both crystals 106, 116 may be attached to the block 102,the base 110, the block 112, or other conductive structure coupled to alower side of the crystals. The fixed mounting block 102 supports theangle block 104. The angle block 104 comprises cooperating mountingsurfaces configured to locate the crystal 106 in a defined location andorientation in the x-y plane, and a defined orientation with respect tothe Y axis. The crystal 106, having a generally cubic or rectangularparallelepiped shape, rests on the mounting surfaces. An upper block 108rests on the crystal 106 opposite the angle block 104. An electrode 126is coupled to the upper block 108 for applying a voltage across the Zaxis of the crystal 106.

An adjustable mounting block 112 may be supported by a flexible web 120formed of a resilient structural material, such as the material of thebase 110 or the block 112, and a screw fastener 122. The web 120 may beconfigured to be relatively flexible along an axis parallel to the Yaxis of the crystals 106, 116, and relatively stiff in the x-z plane.The fastener 122 may pass through a threaded hole in the adjustablemounting block 112 and be rotatably engaged in the base 110. Thefastener 122 may be turned either clockwise or counter-clockwise topivot the adjustable mounting block relative to the Y axis. Aspreviously noted, only a small amount of adjustment is generally neededduring initial configuration of the laser to provide a clean laser pulsewithout sidebands.

It should be apparent that the system 100 is configured such thatturning the adjustment screw 122 causes rotation of crystal 116 and itsassociated mounting elements relative to an adjustment axis Y_(a)defined by the flexible web 120. This Y_(a) axis is offset from theoptical Y axis passing through the crystals 116, 106. The web 120 andthe mounting elements 112, 114 should be configured such that theoptical Y axis and the spaced-apart Y_(a) axis of adjustment areparallel. Adjustments of screw 122 will therefore cause angulardisplacement of crystal 116 relative to both axes Y_(a) and Y′ In thealternative, a Pockels cell system may be arranged such that the axis ofadjustment Y_(a) and the optical axis Y are collinear. (On FIG. 5 Z axisshould be replaced with Y)

An angle block 114 is mounted on the adjustable block 112 and holds thesecond crystal 116 at a defined location and orientation in the x-yplane, and at a defined offset angle relative to the first crystal 106.In the illustrated example, the offset angle is 90°. An upper block 118rests on the crystal 116 opposite to the angle block 114. Voltage isapplied across the Z axis of the crystal 116 using the electrode 124 anda second electrode (not shown) on the upper block 118, coupled to theelectrode 126 via a conductor 128.

FIG. 6 shows an exemplary alternative mounting system 200 for adual-crystal Pockels cell adapted to minimize sidebands in a laser pulseby permitting fine Y axis rotational adjustments while maintaining Xaxis and Z axis alignment. Materials and general construction may be asdescribed in connection with system 100, or should be apparent to one ofordinary skill. The system 200 comprises a base 210 holding an angledsupport block 202. The angled support block 202 may be configured tohold both crystals along a common Y axis, with one crystal rotated aboutthe Y axis by an offset angle. In the alternative, the support block maycomprise two complementary pieces oriented opposite one another, as isshown in FIG. 5. A lower electrode 224 may be coupled to the angledsupport block 202 for application of a control voltage.

The crystals 206, 216 rest in the angled support block 202 as previouslydescribed. The upper blocks 208 and 218 rest on the crystals 206, 216respectively, and are coupled to the electrodes 226 and 228 forapplication of a control voltage. A clamp 204 with a screw 232 holds theupper block 208 and first crystal 206 in place. Likewise, a second clamp214 and screw 230 hold the upper block 218 and second crystal 216 inplace.

Either of the crystals 206 or 218 may be rotated by insertion of a verythin foil shim between the crystal and a face of the angle block 202.For example, a shim 234 may be inserted between the second crystal 216and the angle block 202. The foil thickness and shim shape may beselected by one of ordinary skill to cause a small increment ofrotation, for example, 5 minutes. Multiple shims may be used foradditional rotation. Rotation in an opposite direction may be effectedby shimming against an opposing support surface of the angle block 202.After the desired rotation and resulting clean laser pulse are achieved,the clamps may be secured to hold the crystals in place for operation ofthe laser, and no further adjustment should be necessary.

It should be noted that the methods and systems for Y axis rotationalalignment are not used to achieve a theoretically perfect alignmentaround the Y axis. For example, the methods and systems are not used toachieve a perfect 90° Y axis alignment. Instead, the inventioncontemplates that a small degree of “misalignment” from thetheoretically perfect rotational offset may result in a cleaner laserpulse. The exact amount of misalignment to produce optimum results mayvary depending on individual variations in the crystals used, and thusthe invention provides an empirical approach for determining the optimalY axis offset for any given dual-crystal Pockels cell or othermulti-element optical switch. For example, in an alternative embodimentof the invention, the optimal Y axis offset between any two crystals maybe determined by using the methods disclosed herein to adjust a supportand then measuring the amount of adjustment made. Fixed supports thatprovide this optimal alignment may be constructed to obviate the needfor further adjustment. Such fixed supports having a built-in optimaldegree of Y axis alignment or misalignment should also be consideredwithin the scope of the invention.

Having thus described a preferred embodiment of a system and method forreducing or eliminating sidebands in an ultra-fast laser pulse opticalsystem, it should be apparent to those skilled in the art that certainadvantages of the within system have been achieved. It should also beappreciated that various modifications, adaptations, and alternativeembodiments thereof may be made within the scope and spirit of thepresent invention. For example, a dual-crystal Pockels cell isillustrated, but it should be apparent that the invention may be appliedin other systems using multiple electro-optical materials for additivepolarization. Further, one of ordinary skill may devise other mounts forachieving optimal Y axis offset between cells, including a small degreeof rotational misalignment, and the specific mounting structuresdisclosed herein should be regarded as merely exemplary. The inventionis defined by the following claims.

1. A method of configuring a regenerative laser, the method comprising:placing first and second electro-optical crystals into an amplifyingoptical cavity, each electro-optical crystal being angularly displacedwith respect to the other by an offset angle about an optical axis ofthe optical cavity; directing a seed pulse into the optical cavity;electrically controlling the electro-optical crystals such that the seedpulse is first amplified then emitted from the optical cavity as a laserpulse; observing an intensity profile of the laser pulse; and adjustingthe offset angle by an adjustment amount so as to alter the intensityprofile.
 2. The method of claim 1, further comprising repeating thedirecting, controlling, observing, and adjusting steps so as to minimizesidebands of the intensity profile.
 3. The method of claim 1, whereinthe adjustment amount is substantially less than the offset angle. 4.The method of claim 1, wherein the adjusting step comprises interposinga shim between a surface of the second electro-optical crystal and asurface of the second mount.
 5. The method of claim 1, wherein theadjusting step comprises turning an adjustment screw.